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A Giant Trilobite from the Lynton Formation, North Devon, Indicates a Late Emsian Age

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A Giant Trilobite from the Lynton Formation, North Devon, Indicates a Late Emsian Age A.W.A. Rushton and R.A. Fortey A GIANT TRILOBITE FROM THE LYNTON FORMATION, NORTH DEVON, INDICATES A LATE EMSIAN AGE A.W.A. RUSHTON AND R.A. FORTEY Rushton, A.W.A. and Fortey, R.A. 2018. A giant trilobite from the Lynton Formation, North Devon, indicates a Late Emsian age. Geoscience in South-West England, 14, 194–197. A large and unusually well-preserved trilobite pygidium collected in situ from the Lynton Formation at Lynton, north Devon, is identified as that of the homalonotid Digonus gigas (Roemer), and indicates a late Emsian age for the oldest Devonian strata on the North Devon coast. Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K. (E-mail: [email protected]). Keywords: Devonian, homalonotid trilobite, Rhenish, correlation INTRODUCTION TRILOBITE OCCURRENCE AND ORIGIN The Lynton Formation is displayed in well-exposed, but The trilobite pygidium was recognised as a fossil and often inaccessible cliffs around the town in North Devon that collected during construction work excavated into a fine gives the formation its name. These rocks comprise a folded sandstone bedrock towards the top of the hill at Lynton, high sequence of clastic rocks estimated at between 300 and 400 above the left bank of the West Lyn River (SS 73010 48975). The metres thick (Edmonds et al., 1985; Leveridge, 2011), with the trilobite was brought into the Natural History Museum by its lower part dominated by thick-bedded sandy and silty strata, discoverer, Dr Frederika Holmes, and comprises part and but with argillaceous beds becoming more prominent higher in counterpart of a very large pygidium, which was immediately the sequence. Since Simpson (1964), the Lynton Formation has recognisable as belonging to a homalonotid. Dr Holmes very been accepted as the oldest marine Devonian exposed along kindly donated the specimen to the Museum for scientific study the North Devon coast, lying at the centre of the Lynton after consultation with her family. The comparatively fine Anticline, and surrounded by the prominent sandstones of the preservation of this specimen contrasts with the indifferent Hangman Grits. Despite strenuous attempts to retrieve fossils, condition of fossils from the argillaceous sediments known the palaeontological evidence bearing on the age of the Lynton previously from the same formation. The coarser matrix Formation has remained scrappy, and the long list of fossils in evidently preserved the trilobite from the cleavage and tectonic Edmonds et al. (1985, pp.7–8) includes predominantly tentative distortion that affected the associated shalier rocks. Some determinations. Marine fossils collected from the argillaceous further preparation of the hard sandstone was necessary to beds are generally not well preserved, and are dominated by extract buried parts of the flanks of the specimen and the molluscs with long stratigraphic ranges. None of the standard terminal pygidial spine. The preservation is sufficiently good to Rhenish ammonoid zone fossils have been discovered. The retain some details of the surface sculpture, and allows a best-preserved brachiopods were described by Evans (1983) determination, even without the discovery of corresponding who indicated a late Emsian assemblage, improving upon an cephalic sclerites. Emsian-Eifelian age, close to the Lower to Middle Devonian Pound and Chapman (2004) have argued that some of the boundary, previously tentatively suggested by House et al. folding in the Lynton Formation was penecontemporaneous (1977, table 1, column SW 3). Other than an unlocalised with its deposition, and recognised therein a submarine slide of “Phacops sp.” of little stratigraphical significance, trilobites had considerable magnitude, using structural and cleavage criteria never been recorded. The discovery of a well-preserved as evidence. The direction of movement of the slide was pygidium of a very large trilobite was both unexpected and deduced by these authors to be “towards the SSW”, that is, stratigraphically important. The specimen in question was away from the Bristol Channel. This slumped structure includes excavated from bedrock, and was brought to The Natural sandstone beds similar to those that yielded the trilobite fossil. History Museum, London, in 2016, coming in as an enquiry to It seems plausible to us that the large homalonotid trilobite was the Angela Marmont Centre, from where Fiona Fernhead entrained in a similar slide, accounting for its basin-ward quickly brought it to the attention of the authors. The new emplacement from an originally shallower site. Large early trilobite discovery and its stratigraphical implications are the Devonian homalonotids are typically found in shallow-water subject of this short paper. clastic deposits in the Rhenish ‘magnafacies’, and indeed over a 194 A giant trilobite from the Lynton Formation, North Devon, indicates a Late Emsian age Figure 1. Digonus gigas (Roemer), pygidium. Natural History Museum, NHMUK PI It 29219. 1(A), dorsal view, x1; 1(B), left lateral view, x1; 1(C), posterior view, x1, showing the terminal spine slightly displaced by a crack; 1(D), enlargement of part of Fig. 1B to show the surface sculpture on the lateral parts of the pleural region, x 2.5. 195 A.W.A. Rushton and R.A. Fortey more extended area fringing the Gondwana core (e.g., South show a rough scabrous sculpture that includes a scattering of Africa; Cooper, 1982). They are not typical of more offshore irregularly spaced larger granules (Fig. 1D). facies. Pound and Chapman (2004) surmised that major slides were set off by movements along faults located within the present day Bristol Channel, presumably in connection with an DISCUSSION active phase in the tectonic history of the Rheic Ocean. A landmass in the same general area has been proposed for some The pygidium described here matches well the descriptions time (Bluck et al., 1992) and this area might well have been by both Wenndorf (1990) and Gürich (1909, p. 157, fig. 42, pl. fringed with shallow seas providing suitable habitats for large 48, fig. 3b), though the terminal spine does not curve benthic trilobites like homalonotids; these were periodically downwards as inferred by Gürich, but extends directly caught up in foundering events. However, recent interpretations backwards, as illustrated by Wenndorf (1990, fig. 28). (summarised by Leveridge, in Whittaker and Leveridge, 2011) Comparison with illustrations of German specimens suggests postulate that during the Devonian to Early Carboniferous, the that the three anterior segments of the Lynton specimen are not location of the north Devon area lay some 200 km to the south- preserved (see Fig. 2); if this is correct, the pygidium would, east along the Bristol Channel/Bray fault. The source of both when complete, have had about 11 axial rings and 8 pairs of slumps and trilobite might thus be associated with a different pleural ribs, numbers that are typical for Digonus. The Lynton landmass, far removed from the present-day shelf pygidium does not show the downwardly curved lateral profile configurations. Regardless of the tectonic setting, if we are of the axis, as in several other species (e.g., Digonus correct in identifying the specimen from North Devon with intermedius in Wenndorf, 1990, pl. 13; D. antarcticus Saul Digonus gigas (Roemer, 1843) it adds to the evidence that 1965), and has simple axial rings rather than the “step-like” there was ready marine contact between this part of western rings seen in Digonus of the ornatus group (Kennedy, 1994). England and what is now the Rhenish region of western Preservation is good enough to see the coarse, irregular surface Germany. This distinctive species was widespread there in sculpture typical of the species (Wenndorf, 1990, p. 68), and latest Emsian times. there are no grounds to suppose that the Lynton specimen belongs to any species other than D. gigas, although cephalic information is obviously lacking. THE TRILOBITE FROM LYNTON If it is accepted that three pygidial segments, each 8 or 9 mm We identify the pygidium from the Lynton Formation as a long, have been broken off (Fig. 2), we estimate that the species of the genus Digonus Gürich, 1909, which was original length of the pygidium was originally nearly 120 mm; discussed by Sandford (2005, p. 21). We do not have the following Wenndorf’s (1990, figs 28, 29) reconstruction, this cephalon, but the pygidium corresponds closely to Sandford’s would correspond to a trilobite over 400 mm long, which is diagnosis. Species of Digonus are recorded throughout the commensurate with some German records (Koch 1883), but is Lower Devonian in many parts of the world (Sandford, 2005). substantially larger than the British Devonian homalonotids The type species of Digonus is Homalonotus gigas Roemer, described in Kennedy’s (1994) monograph (with the possible 1843; this species was revised by Wenndorf (1990, p. 64) and exception of a thoracic fragment shown in Kennedy’s plate 4, we accept the synonymy he presented. figure 1). The only other British homalonotid that we know of that may approach the Lynton specimen in size is the Description of the specimen from Lynton Ordovician form Brongniartella? rudis from North Wales, though the deformed condition of Salter’s (1865, pl. 10, fig. 5) The specimen is registered in the NHM collections with the giant specimen prevents accurate comparison. number NHM PI It 29219. Kennedy (1994, pls 3–5) monographed the Devonian The length, as preserved, is 92 mm, but the anterior part of homalonotids of south Devon, nearly all of which were the pygidium is broken off. The maximum width is estimated to collected from the Meadfoot Group and Staddon Grits, and are be 60 mm (doubled half-width) and the maximum height as considered to be of Emsian age. The specimens that he preserved is about 52 mm. The incomplete pygidial axis is assigned to Digonus ornatus disornatus and D.
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